Usage

The immediate use of asn1ate is to generate pyasn1 definitions from
ASN.1 definitions. The command to do this is:

$ python .../asn1ate/pyasn1gen.py source.asn1

It will print the pyasn1 equivalent of source.asn1 to stdout.

But asn1ate is also designed to be usable as a library, to allow reuse of
the ASN.1 parser for custom code generation.

Caveat #2

My understanding of ASN.1 syntax and semantics is limited. As I learn and
discover more, the design will change. I will always prioritize a better model
before API stability for the foreseeable future.

So if you’re using asn1ate as a library for custom codegen, please be aware
that every release will likely break something for you.

The in-tree pyasn1 code generator will be kept up-to-date, of course, and
may serve as an example of different codegen tasks,

Dependencies

The only third-party dependency is pyparsing.

Although asn1ate was initially developed on Python 3.2, it has been tested
with Python 2.7 and should port to older Python versions easily. Latest release
tested with:

Python 3.6.3

Python 2.7.14

Design notes

The asn1ate package is designed along the same lines as a compiler with a
driver, a parser, a semantic model and a convention for code generators.

parser.py – a tokenizing parser for ASN.1 per X.680. It currently
recognizes a naive sub-set of X.680

sema.py – a semantic ASN.1 object model, which can be constructed from
the AST generated by parser.py

support/pygen.py – a support library for generating Python code.

pyasn1gen.py – a code generator to transform a semantic model into
pyasn1 syntax. This can be used as a script in which case it will dump
output to stdout.

The ASN.1 parser is very ad-hoc, I’ve experimented with the grammar until I
found something that accepted our proprietary ASN.1 definition. It’s based on
pyparsing but sets up parse actions to build an annotated AST. Every node of
interest is annotated with a string denoting its type, e.g. Identifier,
TypeAssignment, etc. I’ve tried to stay with token types as named in X.680,
but added custom ones or suppressed others, as necessary to get the AST in a
useful shape.

Annotated tokens are represented by a simple class containing the type name and
a list of children (called elements) which may be annotated tokens, lists or
simple values. This gives a very discoverable tree structure, but there are
probably cleaner AST representations we could use. Patches welcome.

asn1ate.sema is an object model that represents ASN.1 constructs. It
describes everything from type assignments to default values and tags, but still
only the parts of ASN.1 we happen to use here. Most of the logic revolves around
transforming the AST produced by asn1ate.parser into a more semantic model
with proper Python objects.

Codegen is designed to be extensible. In-house we have a set of code generators
to build an entire protocol stack based on an ASN.1 source, but asn1ate only
includes the generally useful one, asn1ate.pyasn1gen.

The most notable members of asn1ate.support are probably the
PythonWriter and PythonFragment classes, which simplify generation of
correctly indented Python code.